Generally, flue gas desulfurization (FGD) systems can be categorized as being either a dry system or a wet system. Dry systems collect a dry pollutant from the exiting gas stream such as by the use of mechanical collectors, fabric filters or the like. Wet systems produce a liquid slurry that must be further dewatered before the end product and/or pollutant can be disposed of. Because of the close contact between the pollutant and the liquid slurry in wet systems, additional process functions can also be performed than is possible with dry systems. These functions include gas absorption, chemical reaction and heat transfer along with the simultaneous removal of dust and gaseous pollutants by use of a suitable scrubbing solution.
In perhaps the majority of wet scrubbing systems, the scrubbing solution is calcium based (such as lime, limestone, or slaked lime). In others it may be magnesium or sodium based, or even ammonia based, depending upon the pollutant to be removed and the costs involved. Additionally, it is common to supply additives or catalysts to the scrubber tower to further enhance sulfur removal.
However, despite the variety of systems available, it has not heretofore been known that certain such systems may be combined to further improve the operation of the scrubber tower. By such a combination, the advantages of one system are used to overcome the disadvantages of the other system. Consequently, not only is a greater percentage of the sulfur or other contaminants removed, but such removal is accomplished at a lower cost and greater efficiency. Additionally, the end product of such a system need not be sent to a landfill for disposal, instead the end product can be utilized in other industries thereby possibly generating revenue rather than being an expense.
It is thus an object of this invention to combine calcium and ammonia based reagents in a wet scrubber tower to promote a high degree of sulfur removal from the flue gas. Another object of this invention is to implement forced oxidation in the scrubber tower to convert SO.sub.3 (sulfite) to SO.sub.4 (sulfate) for subsequent combination with calcium to formulate gypsum (CaSO.sub.4.2H.sub.2 O). Yet another object of this invention is to utilize ammonia to generate an end product that is useful in agriculture such as a soil stabilizer or fertilizer. Such agricultural product may be in wet or dry form depending on its use. Still another object of this invention is to utilize the lower cost limestone rather than the more expensive lime as the calcium source in the scrubber solution. Yet another object of this invention is to reduce corrosion in the scrubber tower by maintaining a higher pH therein. Another object of this invention is to employ a system that can be used in a spray tower or a tray tower either of which may be of a single or dual loop design. Still another object of this invention is to provide a staged tray tower wherein the top stage or section consists predominantly of an ammonia scrubbing loop. A further object of this invention is to increase the SO.sub.2 removal efficiency of 95% or more by using an NH.sub.3 additive in lime/limestone scrubbing at higher pH levels of 5.5-6.0 instead of lower pH levels of 4.5-5.0 for buffered systems using organic additives in a tray tower. These and other objects and advantages of this invention will become obvious upon further investigation.